Method for operating an energy supply unit for a motor vehicle electrical system

ABSTRACT

A method for operating an energy supply unit for a motor vehicle electrical system, including at least one first subsystem and one second subsystem having different voltage levels, the energy supply unit including an electric machine which is connected via a converter circuit to the first subsystem and the second subsystem. In a first operating mode, a switchable switch element of the converter circuit which connects the converter circuit to the second subsystem is opened, the converter circuit is activated as an inverter circuit and the electric machine is motor or generator operated. In a second operating mode, the switchable switch element of the converter circuit is closed, the converter circuit is activated as a DC-DC converter and the DC-DC conversion takes place between the voltage levels of the first and the second subsystem.

RELATED APPLICATION INFORMATION

The present application claims priority to and the benefit of Germanpatent application No. 10 2013 206 296.6, which was filed in Germany onApr. 10, 2013, the disclosure of which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to a method for operating an energy supplyunit for a motor vehicle electrical system, including at least one firstsubsystem and one second subsystem having different voltage levels.

BACKGROUND INFORMATION

Motor vehicle electrical systems may be configured as so-calledtwo-voltage or multi-voltage vehicle electrical systems including atleast two subsystems. Such electrical systems are used, for example,when consumers having different power requirements exist in a particularmotor vehicle. In this case, at least two of the subsystems havedifferent voltage levels, for example, 14 V (a so-called low-voltagesubsystem) and 48 V (a so-called high-voltage subsystem). The subsystemsmay be connected to each other, for example via a DC-DC converter. Atleast one of the subsystems has a generator system that feeds thesubsystem. A second or additional subsystem connected via the mentionedDC-DC converter may then in turn be supplied from the subsystem havingthe generator system.

Electric machines may be used, in particular, in hybrid vehicles inorder to be motor operated as well as generator operated. The internalcombustion engine may be assisted by a motor operation of the electricmachine at low rotational speeds at which the former does not yetdeliver its full torque. Upon deceleration of the motor vehicle, kineticenergy may then be converted into electrical energy by the generatoroperation of the electric machine.

During generator operation, the electric machine generates, ifnecessary, a polyphase current which may be rectified for a motorvehicle electrical system. To enable both motor operation as well asgenerator operation of the electric machine, the electric machine may beequipped with an inverter circuit which may be composed, for example, ofelectrical switches, for example, in the form of MOSFETs, an associatedcontrol circuit and an intermediate capacitance. To ensure highperformances in both motor as well as generator operation of theelectric machine, the electric machine may be operated with, or it maysupply, the comparatively high, first voltage of the high voltagesubsystem.

However, the use of both an inverter circuit and a DC-DC converter inthis configuration is cumbersome and is associated with high costs.Moreover, the separate circuits of the inverter circuit and the DC-DCconverter put a strain on the already severely limited installationspace in a motor vehicle.

It is therefore desirable to provide a simple, cost-efficient andspace-saving option for enabling both a generator as well as a motoroperation of an electric machine in conjunction with differentsubsystems of the motor vehicle electrical system.

SUMMARY OF THE INVENTION

The present invention provides a method for operating an energy supplyunit for a motor vehicle electrical system having the features describedherein. Advantageous embodiments are the subject matter of the furtherdescriptions, as well as the following description.

The energy supply unit includes an electric machine to which an invertercircuit is connected which, in turn, is connected to one first subsystemand via a switch element to one second subsystem of a multi-voltagevehicle electrical system. Through the use according to the presentinvention of the converter circuit equipped with such a switch element,the energy supply unit may be operated or activated in a first operatingmode as an inverter circuit. In this case the converter circuit has thesame functions and advantages as a conventional inverter circuit and, inparticular, may be configured analogously to a conventional invertercircuit. This makes possible both a generator operation of the electricmachine, the electric machine supplying the first subsystem of the motorvehicle electrical system, as well as a motor operation of the electricmachine, the electric machine being supplied from the first subsystem.

In a second operating mode the converter circuit is operated oractivated as a DC-DC converter. In this case the same components of theconverter circuit and, if necessary, the electric machine are used andare activated in such a way that a DC-DC conversion takes place betweenthe voltage levels of the first subsystem and the second subsystem.According to the present invention, the already existing parts andcomponents of the converter circuit which, in particular, are those of aconventional inverter circuit, are accordingly also used for the DC-DCconversion. Therefore, no additional components and parts are requiredand the costs may be reduced. The costs of integration and spacerequirements are also reduced.

Accordingly, the energy supply unit according to the present inventionenables both a generator operation as well as a motor operation of theelectric machine and enables the operation of multiple subsystems of themotor vehicle electrical system. Thus, the energy supply unit accordingto the present invention combines the advantages and functions of aninverter circuit and a DC-DC converter in one single circuit.

A processing unit used for activating the converter circuit, including,for example, a microcontroller, may be used for controlling therectification and the inversion as well as for controlling the DC-DCconversion. Moreover, this enables the electric machine to transferelectrical power directly into the first subsystem as well as into thesecond subsystem. This is particularly advantageous during an emergencyoperation of the generator in the event of a battery failure.

The present invention is particularly suited for electric machines, forexample, a separately excited synchronous machine for use in motorvehicles. The principle may be employed in connection with a boostrecuperation system (BRS) in the electric machine (boost recuperationmachine).

A processing unit according to the present invention, for example, acontrol unit of a motor vehicle, is programmed, in particular, to carryout a method according to the present invention. The processing unit,together with the electric machine, which may form a structural unit inorder to collectively form an “intelligent” electric machine.

The implementation of the method in the form of software is alsoadvantageous, since this entails particularly low costs, in particularif a performing control unit is also used for other tasks and istherefore present anyway. Suitable data media for providing the computerprogram are, in particular, diskettes, hard-disk drives, flash memories,EEPROMs, CD-ROMs, DVDs and the like. It is also possible to download aprogram from computer networks (Internet, Intranet etc.).

Further advantages and embodiments of the present invention result fromthe description and the appended drawing.

It is understood that the features cited above and those to be explainedbelow are applicable not only in each specified combination, but also inother combinations or alone, without departing from the scope of thepresent invention.

The present invention is schematically represented in the drawing basedon exemplary embodiments and is described in greater detail below withreference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows one specific embodiment of a multi-voltagevehicle electrical system having an energy supply unit according to therelated art.

FIG. 2 shows one specific embodiment of a multi-voltage vehicleelectrical system having an energy supply unit which is configured tocarry out one specific embodiment of a method according to the presentinvention.

FIG. 3 shows in a circuit diagram-like manner one specific embodiment ofan energy supply unit which is configured to carry out one specificembodiment of a method according to the present invention.

FIG. 4 shows in a circuit diagram-like manner a charge phase (FIG. 4a )and a discharge phase (FIG. 4b ) of a DC-DC conversion of an energysupply unit according to one specific embodiment of a method accordingto the present invention.

FIG. 5 schematically shows two diagrams which may be determined in thecourse of a charge phase and a discharge phase of a DC-DC conversion ofan energy supply unit according to one specific embodiment of a methodaccording to the present invention.

FIG. 6 shows in a circuit diagram-like manner one specific embodiment ofan energy supply unit having three switch elements, which is configuredto carry out another specific embodiment of a method according to thepresent invention.

FIG. 7 shows in a circuit diagram-like manner another specificembodiment of an energy supply unit having a five-phase electric machinewhich is configured to carry out another specific embodiment of a methodaccording to the present invention.

DETAILED DESCRIPTION

Corresponding elements are denoted by identical reference numerals. Forthe sake of clarity, these will not be repeatedly explained.

FIG. 1 schematically shows one specific embodiment of a multi-voltagevehicle electrical system having an energy supply unit of a motorvehicle electrical system according to the related art. In this example,the motor vehicle is configured as a hybrid vehicle. Connecteddownstream from an electric machine 100 is an inverter circuit 150. Inthis example, electric machine 100 is intended to be configured as athree-phase electric machine 100. Inverter circuit 150 is used torectify a multiphase current, in this example, a three-phase currentwhich is provided by electric machine 100 during a generator operation.In addition, inverter circuit 150 enables a conversion of a rectifiedcurrent into a three-phase current in order to operate electric machine100 in a motor mode.

During the generator operation of electric machine 100, inverter circuit150 provides a first subsystem d.c. voltage of, for example, 48 V for afirst subsystem N₁ of the motor vehicle electrical system. With the aidof this first subsystem d.c. voltage, it is possible to operate multipleelectrical consumers, which are represented symbolically in FIG. 1 anddesignated as V₁ and V₂. Such an electrical consumer may, for example,be an electric drive of the hybrid vehicle or an energy storerepresented as V₂.

Since most electrical components in the hybrid vehicle, such as astarter motor of an internal combustion engine, a car radio or anon-board computer, are operated with a lower voltage than the firstsubsystem d.c. voltage, the first subsystem d.c. voltage is reduced by aDC-DC converter to a second subsystem d.c. voltage, for example, 14 V,for a second subsystem N₂. Electrical components which are operated withthe second subsystem d.c. voltage are represented symbolically in FIG. 1and designated as V₃, V₄ and V₅.

The voltage values 48 V and 14 V used are merely examples. The presentinvention may also be used in conjunction with other voltages orvoltages varying over time.

FIG. 2 schematically shows one specific embodiment of a multi-voltagevehicle electrical system having an energy supply unit 1, which isconfigured to carry out one specific embodiment of a method according tothe present invention. Connected downstream from an electric machine 100is a converter circuit 200. Converter circuit 200 is used, on the onehand, as an inverter circuit in order to provide the first subsystemd.c. voltage at a first d.c. voltage terminal U₁, with which electriccomponents V₁ and V₂ may be operated.

Converter circuit 200 is used, on the other hand, as a DC-DC converterin order to transform the first subsystem d.c. voltage into the secondsubsystem d.c. voltage, with which components V₃, V₄, V₅ are operated,and to provide the second subsystem d.c. voltage at second d.c. voltageterminal U₂. In addition, inverter circuit 150 may transfer electricalenergy between the two motor vehicle electrical systems with the firstsubsystem d.c. voltage and with the second subsystem d.c. voltage.

Energy supply unit 1 and a specific embodiment of a method according tothe present invention for operating energy supply unit 1 are describedwith reference to FIG. 3.

Electric machine 100 in this example is configured as a three-phaseelectric machine. Stator inductances (phases) L₁, L₂ and L₃ of electricmachine 100 are connected to a delta circuit. It may be noted thatelectric machine 100 includes still other components, for example, arotor or an excitation winding, which for the sake of clarity are notshown.

Converter circuit 200 includes three half bridges B₁, B₂ and B₃. Each ofthe three half bridges B₁, B₂ and B₃ includes two switches S₁, S₄ andS₂, S₅, and S₃, S₆, respectively. Switches S₁ through S₆ may beconfigured as MOSFETs, for example. Each half bridge B₁, B₂ and B₃includes in each case a center tap M₁, M₂ and M₃ between their twoswitches. Each of half bridges B₁, B₂ and B₃ is connected via itsrespective center tap M₁, M₂ and M₃ to one of the phase connections E₁,E₂ and E₃ of electric machine 100. Half bridges B₁, B₂ and B₃ areconnected on the output side to a d.c. voltage terminal U₁ of firstsubsystem N₁ and an earth terminal U₀. In addition, an intermediatecapacitance C₁ is connected in parallel to half bridges B₁, B₂ and B₃.

The above-described part of converter circuit 200 is configuredanalogously to an inverter circuit 150 according to the related art. Thephases of electric machine 100 are energized as a result of the clockedswitching of switches S₁ through S₆. During motor operation of electricmachine 100, electrical power from first subsystem N₁ is converted intomechanical power by correspondingly activating switches S₁ through S₆.During generator operation, mechanical power is converted intoelectrical power and is delivered to first subsystem N₁.

In terms of the present invention, converter circuit 200 includes aswitch element S* via which a center tap of one first half bridge of thehalf bridges is connected to a second d.c. voltage terminal U₂ of secondsubsystem N₂. In this specific case, switch element S* is connected tocenter tap M₁ of half bridge B. In addition, a smoothing capacitor C* isconnected on the output side in parallel between second d.c. voltageterminal U₂ and earth terminal U₀.

With the advantageous use of switch element S*, it is possible to useconverter circuit 200 not only as an inverter circuit, but as a DC-DCconverter as well. In this arrangement, electrical power may betransferred from first subsystem N₁ into second subsystem N₂ and viceversa.

If switch element S* is opened, converter circuit 200 is then used as aninverter circuit and the first subsystem d.c. voltage is providedbetween first d.c. voltage terminal U₁ and earth terminal U₀.

If switch element S* is closed, the second subsystem d.c. voltage isthen provided between second d.c. voltage terminal U₂ and earth terminalU₀. In this case, smoothing capacitor C* as well as two advantageouslyactivated switches of switches S₁ through S₆ form a DC-DC converter.

Advantageously, the two switches of one of the half bridges which arenot connected to switch element S* are selected and activated. A chargephase alternates with a discharge phase of a coil as a result of theclocked activation of the respective switches.

This coil is advantageously configured as one of the three statorinductances L₁, L₂, L₃ of electric machine 100. Thus, it is notnecessary to integrate an additional coil into energy supply unit 1, andno additional components are required. Already existing stator windingsL₁ through L₃ of electric machine 100 are accordingly used as a coil ofthis DC-DC converter.

Shown in addition to energy supply unit 1 is a processor unit which isconfigured, in particular, as a control unit 300 of the vehicle, whichis programmed to carry out a specific embodiment of a method accordingto the present invention. Control unit 300 controls the activation ofelectric machine 100 and converter circuit 200 in general, and of theindividual parts and the switching of individual switches S₁ through S₆and of switch element S* in particular. Processor unit 300 is acomponent of electric machine 100 and together with the latter andconverter circuit 200 forms a structural unit.

The activation of the respective switches and the charge phase anddischarge phase are explained with reference to FIGS. 4 and 5. FIGS. 4aand 4b show energy supply unit 1 from FIG. 3. For the sake of clarity,only those reference numerals are shown in FIGS. 4a and 4b which are ofsignificance for the charge phase and the discharge phase. In theexample in FIG. 4, switches S₂ and S₅ of half bridges B₂ are activated.Switches S₂ and S₅, smoothing capacitor C* and stator winding L₁ areused as a DC-DC converter.

The charge phase of energy supply unit 1 is shown in FIG. 4a .Current-carrying conductors are highlighted in bold. In this example,switch element S* and switch S₂ are closed; the remaining switches areopened. A current having a current intensity I₁ may flow from first d.c.voltage terminal U₁ via switch S₂, center tap M₂, and phase connectionE₂ to stator inductance L₁ of electric machine 100. A current having acurrent intensity I₂ flows from stator inductance L₁ via phaseconnection E₁ and switch element S* into smoothing capacitor C*. Acurrent having a current intensity I₃ flows to second d.c. voltageterminal U₂. Stator inductance L₁ is charged in this charge phase.

Shown in FIG. 4b is the discharge phase of energy supply unit 1. In thiscase, switch S₂ is opened and switch S₅ is closed. Switch element S*remains closed; the remaining switches are opened. The first subsystemd.c. voltage of first subsystem N₁ is no longer present at statorinductance L₁. Stator inductance L₁ maintains the current flow and thendischarges. A current having a current intensity I₄ flows from earthterminal U₀ via switch S₅, center tap M₂ and phase connection E₂ tostator inductance L. The current having current intensity I₂ or I₃ flowsanalogously to FIG. 4 a.

The second subsystem d.c. voltage of second subsystem N₂ occurring atsecond d.c. voltage terminal U₂ may be controlled by the ratio of thedurations of the charge phase and the discharge phase and therefore byclocked activation of switches S₂ and S₅. Smoothing capacitor C* is usedto smooth the second subsystem d.c. voltage.

In upper diagram 31 in FIG. 5 a curve of a current intensity I isplotted against time t. At point in time t₀, the charge phase begins,switch S₂ is closed and switch S₅ is opened. In this case, the dottedline describes the current having current intensity I₁. At point in timet₁, the discharge phase begins, switch S₂ is opened and switch S₅ isclosed. The current having current intensity I₁ is no longer able toflow. Since stator inductance L₁ maintains the current flow, the currenthaving current intensity I₄ described by the dashed-dotted line nowflows. At point in time t₂, the charge phase begins again, analogouslyto point in time t₀. At point in time t₃, the discharge phase beginsagain, analogously to point in time t₁. The solid line describes thecurrent having current intensity I₂ which flows during the charge phaseas well as during the discharge phase. The dashed line describes thecurrent having current intensity I₃ which flows to second d.c. voltageterminal U₂ during the charge phase and the discharge phase. Currentintensity I₃ may be held essentially constant during the charge phaseand the discharge phase by smoothing capacitor C*.

In diagram 32 a curve of a voltage U is plotted against time t. Thesecond subsystem d.c. voltage provided at second d.c. voltage terminalU₂ during the charge phase and the discharge phase is represented as asolid line and denoted by U_(B). The dashed line describes a voltagesetpoint value U* of the second subsystem d.c. voltage, for example, 14V. Second subsystem d.c. voltage U_(B) is smoothed by smoothingcapacitor C* so that the voltage value of second subsystem d.c. voltageU_(B) differs only negligibly from voltage setpoint value U* during thecharge phase and the discharge phase.

It is also conceivable to connect a free-wheeling diode in parallel toswitch S₅. In this case, clocked activation of switch S₅ is notnecessary. If switch S₅ is configured as a MOSFET, for example, thensuch a free-wheeling diode is present as a matter of principle. Byanalogy, this applies to switches S₄ and S₆.

A reversal of the direction of the power flow, i.e., a power transferfrom the second subsystem d.c. voltage to the first subsystem d.c.voltage, may be achieved by closing switch element S* and bycontrastingly opening and closing switches S₂ and S₅.

FIG. 6 schematically shows another embodiment of an energy supply unit1, which is configured to carry out another specific embodiment of amethod according to the present invention.

The present invention is not limited to a switchable switch element. Aconverter circuit 200 may also include multiple switchable switchelements. In this configuration in FIG. 6 each of the 3 half bridges B₁,B₂ and B₃ is connected in each case via their center tap M₁, M₂ and M₃to second d.c. voltage terminal U₂, in each case via a switch elementS₁*, S₂* and S₃*. Each of the phase connections E₁, E₂ and E₃ isconnected therefore in each case via a switch element S₁*, S₂* and S₃*to second d.c. voltage terminal U₂.

In this way, the power to be transferred may be uniformly distributed toswitches S₁ through S₆ and to stator inductances L₁ through L₃ in such away that the thermal stresses on the individual parts resulting frompower dissipation is reduced. Switch elements S₁* through S₃* are closedat staggered intervals. While one of switch elements S₁* through S₃* isclosed, the switch pair of one of the half bridges not connected to theclosed switch element is activated in accordance with the principledescribed above. If, for example, switch element S₃* is closed, switchesS₁ and S₄ of half bridge B₁ or switch S₂ and S₅ of half bridge B₂ may beactivated.

In this way, the switches may be “rollingly” activated so that theirthermal stress is reduced. Another advantage of this arrangement is thatit is possible to operate energy supply unit 1 directly with both firstsubsystem N₁ as well as with second subsystem N₂. In phases ofcontinuous generator supply of second subsystem N₂, the additionalvoltage transformation is therefore omitted, as a result of which powerlosses may be reduced.

FIG. 7 schematically shows another embodiment of an energy supply unit 1which is configured to carry out one specific embodiment of a methodaccording to the present invention.

The use of the present invention is not limited to 3-phase electricmachines. FIG. 7 shows an energy supply unit 1′ according to the presentinvention having a 5-phase electric machine 100′ including five statorinductances L₁′, L₂′, L₃′, L₄′ and L₅′. In this example, the 5-phaseelectric machine 100′ is configured as a drude's foot circuit.

Converter circuit 200′ includes five half bridges B₁′, B₂′, B₃′, B₄′ andB₅′. Each of the five half bridges B₁′ through B₅′ includes in each casetwo switches S₁′ through S₁₀′. Each of half bridges B₁′ through B₅′ isconnected in each case via a center tap M₁′, M₂′, M₃′, M₄′ and M₅′ to aphase connection E₁′. E₂′, E₃, E₄′ and E₅′ of electric machine 100′.

Analogously to FIG. 4, each of phase connections E₁′, E₂′, E₃, E₄′ andE₅′ is connected via respective center tap M₁′, M₂′, M₃′, M₄′ and M₅′ ofthe associated half bridge B₁′, B₂′, B₃′, B₄′ and B₅′ to second d.c.voltage terminal U₂ via a switch element S₁*, S₂*, S₃*, S₄* and S₅*. Theswitches are activated analogously to the case of a three-phase electricmachine 100 described above.

What is claimed is:
 1. A method for operating a motor vehicle electricalsystem that includes (a) a circuit that includes a switch, (b) anelectric machine, (c) a terminal to a first subsystem, and (d) aterminal to a second subsystem that operates at a different voltagelevel than the first subsystem, the method comprising: in a firstoperating mode, in which the switch is closed, the circuit operating asa DC-DC converter that converts between the voltage levels of the firstsubsystem and the second subsystem by controlling a connection via theswitch between the first and second subsystems, wherein the secondsubsystem is connectable to the electric machine via the switch when,and only when, the switch is closed, and the second subsystem isconnectable to the first subsystem via the switch when, and only when,the switch is closed; and in a second operating mode, in which theswitch is open and while the second subsystem is disconnected from thefirst subsystem and from the electric machine, the circuit operating theelectric machine as a motor using electrical energy from the firstsubsystem connected to the electric machine via the circuit, andoperating the electric machine as a generator to supply electricalenergy to the first subsystem connected to the electric machine via thecircuit.
 2. The method of claim 1, wherein, in the first operating mode,the conversion between the voltage levels includes the circuitconnecting the first subsystem to the second subsystem via the electricmachine.
 3. The method of claim 1, wherein, in the second operatingmode, electrical power is transferred between the first subsystem andthe electric machine via the circuit.
 4. The method of claim 1, wherein,in the first operating mode, electrical power is transferred between thefirst subsystem and the second subsystem via the circuit activated asthe DC-DC converter.
 5. The method of claim 3, wherein the transferredelectrical powers are controlled by clocked activation of switches ofthe circuit.
 6. The method of claim 1, wherein, in the second operatingmode, a multiphase output voltage of the electric machine is rectifiedinto a first subsystem d.c. voltage of the first subsystem or the firstsubsystem d.c. voltage of the first subsystem is inverted into amultiphase voltage.
 7. The method of claim 1, wherein, in the firstoperating mode, the first subsystem d.c. voltage of the first subsystemis transformed into a first subsystem d.c. voltage of the firstsubsystem via the circuit activated as the DC-DC converter, or thesecond subsystem d.c. voltage of the second subsystem is transformedinto the first subsystem d.c. voltage of the first subsystem via thecircuit activated as the DC-DC converter.
 8. The method of claim 7,wherein voltage values of the transformed subsystem d.c. voltages arecontrolled by clocked activation of the switches of the circuit.
 9. Themethod of claim 1, wherein, in the first operating mode, switches of thecircuit and a stator inductance of the electric machine are operated asthe DC-DC converter.
 10. The method of claim 1, wherein the circuitincludes at least two switchable switch elements, the at least twoswitchable switch elements being opened in the second operating mode,and the at least two switchable switches being closed in staggeredintervals in the first operating mode.
 11. The method of claim 10,wherein each of the at least two switchable switch elements is connectedto a respective center tap of a respective half bridge of the circuit.12. A processor unit, comprising: an arrangement, wherein thearrangement is configured for operating a motor vehicle electricalsystem by performing the following: (I) in a first operating mode, inwhich a switch of a circuit of the electrical system is closed,controlling the circuit to operate as a DC-DC converter that convertsbetween (a) a first voltage level at which a first subsystem, a terminalto which is included in the electrical system, operates and (b) a secondvoltage level that is different than the first voltage level and atwhich a second subsystem, a terminal to which is included in theelectrical system, operates, wherein: the controlling of the circuit tooperate as the DC-DC converter is by controlling a connection via theswitch between the first and second subsystems; and the second subsystemis connectable to an electric machine of the electrical system via theswitch when, and only when, the switch is closed, and the secondsubsystem is connectable to the first subsystem via the switch when, andonly when, the switch is closed; and (II) in a second operating mode, inwhich the switch is open and while the second subsystem is disconnectedfrom the first subsystem and from the electric machine, controlling thecircuit to: operate the electric machine as a motor using electricalenergy from the first subsystem connected to the electric machine viathe circuit; and operate the electric machine as a generator to supplyelectrical energy to the first subsystem connected to the electricmachine via the circuit.
 13. A non-transitory computer readable mediumon which is stored a computer program that is executable by a processorand that, when executed by the processor, causes the processor toperform a method for operating a motor vehicle electrical system, theelectrical system including (a) a circuit that includes a switch, (b) anelectric machine, (c) a terminal to a first subsystem, and (d) aterminal to a second subsystem that operates at a different voltagelevel than the first subsystem, the method comprising performing thefollowing: in a first operating mode, in which the switch is closed,controlling the circuit to operate as a DC-DC converter that convertsbetween the voltage levels of the first subsystem and the secondsubsystem by controlling a connection via the switch between the firstand second subsystems, wherein the second subsystem is connectable tothe electric machine via the switch when, and only when, the switch isclosed, and the second subsystem is connectable to the first subsystemvia the switch when, and only when, the switch is closed; and in asecond operating mode, in which the switch is open and while the secondsubsystem is disconnected from the first subsystem and from the electricmachine, controlling the circuit to: operate the electric machine as amotor using electrical energy from the first subsystem connected to theelectric machine via the circuit; and operate the electric machine as agenerator to supply electrical energy to the first subsystem connectedto the electric machine via the circuit.
 14. The computer readablemedium of claim 13, wherein, in the second operating mode, electricalpower is transferred between the first subsystem and the electricmachine via the circuit.
 15. A vehicle electrical system, comprising: aterminal to a first subsystems; a terminal to a second subsystem thatoperates at a different voltage level than the first subsystem; acircuit that includes a switch; an electric machine; and a processorunit, wherein the processor unit is configured to operate the electricalsystem by performing the following: in a first operating mode, in whichthe switch is closed, controlling the circuit to operate as a DC-DCconverter that converts between the voltage levels of the firstsubsystem and the second subsystem by controlling a connection via theswitch between the first and second subsystems, wherein the secondsubsystem is connectable to the electric machine via the switch when,and only when, the switch is closed, and the second subsystem isconnectable to the first subsystem via the switch when, and only when,the switch is closed; and in a second operating mode, in which theswitch is open and while the second subsystem is disconnected from thefirst subsystem and from the electric machine, controlling the circuitto: operate the electric machine as a motor using electrical energy fromthe first subsystem connected to the electric machine via the circuit;and operate the electric machine as a generator to supply electricalenergy to the first subsystem connected to the electric machine via thecircuit.